It is well known in the art to plate non-conductive substrates, (i.e. plastics) with metal for a variety of purposes. Plastic moldings are relatively inexpensive to produce and metal plated plastic is used for many applications. For example, metal plated plastics are used for decoration and for the fabrication of electronic devices. An example of a decorative use includes automobile parts such as trim. Examples of electronic uses include printed circuits, wherein metal plated in a selective pattern comprises the conductors of the printed circuit board, and metal plated plastics used for EMI shielding. ABS resins are the most commonly plated plastics for decorative purposes while phenolic and epoxy resins are the most commonly plated plastics for the fabrication of printed circuit boards.
Preparing plastics for subsequent plating is a multi-step process and typical process steps include:                1) etching the substrate with a chromic acid etching solution;        2) neutralizing the etched surface with a chrome neutralizing solution;        3) activating the etched surface using a colloidal palladium tin activator;        4) removing tin with an accelerating step; and        5) depositing a layer of electroless copper or electroless nickel followed by electrolytic copper and/or nickel plating.        
The initial etching of the plastic substrates is an essential part of the overall process, and essentially all commercial processes have utilized a chromic acid etch solution as a source of hexavalent chromium for the plastic etching step. This process has many attributes. Various plastics including ABS and ABS/polycarbonate blends can be plated with good plate appearance and adhesion. Immersion time and/or temperature in the chromic acid etch solution can be increased to plate more difficult plastics containing higher levels of polycarbonate or polypropylene. Extremely difficult plastics that are etch resistant, such as pure polycarbonate, can also be plated by incorporating a solvent prior to the chromium etching step.
Only certain types of plastic components are suitable for plating, and, as discussed above, the most common types of plastic for electroplating are acrylonitrile/butadiene/styrene (ABS) or a blend of this material with polycarbonate (ABS/PC). ABS consists of two phases. There is a relatively hard phase consisting of an acrylonitrile/styrene copolymer and a softer polybutadiene phase. Currently, this material is etched almost exclusively using a mixture of chromic and sulfuric acids. This oxidizing acid mixture is highly effective as an etchant for ABS and ABS/PC. The polybutadiene phase of the plastic contains double bonds in the polymer backbone and these are oxidized by the chromic acid thus causing complete breakdown and dissolution of the polybutadiene phase exposed at the surface of the plastic thus giving an effective etch to the surface of the plastic.
The purpose of the etching step is two fold. First, the plastic is etched to increase surface area. Secondly, the plastic is made hydrophilic, making the surface receptive to subsequent activating and plating stages. Typical chromic acid etching solutions are described, for example, in U.S. Pat. No. 4,610,895 to Tubergen et al., U.S. Pat. No. 6,645,557 to Joshi and U.S. Pat. No. 3,445,350 to Klinger et al., which are herein incorporated by reference in their entirety.
One problem with the traditional chromic acid etching step is that chromic acid is a recognized carcinogen and is increasingly regulated, insisting that wherever possible, the use of chromic acid is replaced with safer alternatives. The use of a chromic acid etchant also has well-known and serious drawbacks, including the toxicity of chromium compounds which makes their disposal difficult, chromic acid residues remaining on the polymer surface that inhibit electroless deposition, and the difficulty of rinsing chromic acid residues from the polymer surface following treatment. Additionally, hot hexavalent chromium sulfuric acid solutions are naturally hazardous to workers. Burns and upper respiratory bleeding are common in workers routinely involved with these chrome etch solutions. Thus, it is very desirable that safer alternatives to acidic chromium etching solutions be developed.
Permanganate solutions are described in U.S. Pat. No. 3,625,758 to Stahl et al., which is herein incorporated by reference in its entirety. Stahl suggests the suitability of either a chrome and sulfuric acid bath or a permanganate solution for preparing the surface.
U.S. Pat. No. 4,948,630 to Courduvelis et al., which is herein incorporated by reference in its entirety, describes a hot alkaline permanganate solution that also contains a material, such as sodium hypochlorite, that has an oxidation potential higher than the oxidation potential of the permanganate solution and is capable of oxidizing manganate ions to permanganate ions. U.S. Pat. No. 5,648,125 to Cane, which is herein incorporated by reference in its entirety, describes the use of an alkaline permanganate solution comprising potassium permanganate and sodium hydroxide, wherein the permanganate solution is maintained at an elevated temperature, i.e., between about 165° F. and 200° F. U.S. Pat. No. 4,042,729 to Polichette et al., which is herein incorporated by reference in its entirety describes an etching solution that comprises water, permanganate ion, and manganate ion, wherein the molar ratio of manganate ion to permanganate ion is controlled and the pH of the solution is maintained at 11-13.
U.S. Pat. No. 5,229,169 to Chao, which is herein incorporated by reference in its entirety, describes a process for depositing a metal layer on the surface of a polycarbonate-ABS resin (or other similar resin) comprising the steps of contacting the surface with an aqueous metal hydroxide solution, contacting the surface with an aqueous alkaline solution of a water-soluble permanganate, removing any residue of manganese compounds by contact with a reducing agent, and depositing an electroless metal layer on the surface. The alkaline permanganate generally comprises sodium or potassium permanganate and the reducing agent may comprise, for example, a solution of hydroxylamine salts.
However, attempts to use permanganate for etching plastics (other than epoxy base printed circuit boards) have not had much success. First, the surface treatment of the plastic is inconsistent, sometimes yielding good adhesion and sometimes yielding poor adhesion under identical treatment conditions. Second, permanganate solutions can be unstable, have a short life and are rapidly decomposed to manganese dioxide. Furthermore, as compared to chrome etchants, permanganate is less effective and not suitable for the wide range of plastic mixtures plated in general metal finishing operations.
None of these attempts to etch plastic using permanganate ions have been capable of producing etch characteristics which match those obtained by the use of chromic acid and the stability of the etching solutions is also poor, resulting in the formation of manganese dioxide sludge.
Other attempts to replace the chrome etching are also described in the prior art. For example, U.S. Pat. Nos. 4,941,940, 5,015,329, and 5,049,230, all to Patel et al., which are herein incorporated by reference in their entirety, describe a single or multi-step process for pre-swelling and etching of functionalized polymers, such as polycarbonates, using an etching solution that comprises at least one swelling agent and at least one degradation agent. The prepared substrates are then plated with electroless nickel or electroless copper.
U.S. Pat. No. 5,160,600 to Patel et al., which is herein incorporated by reference in its entirety, replaces the chromic acid etching solution with an etching solution that comprises sulfuric acid, and optionally phosphoric acid and/or nitric acid. The treated substrate is then immersed in an aqueous suspension of palladium.
Regardless of whether the oxidant solution is a hexavalent chromium solution or a permanganate solution, contact with the solution leaves an oxidant residue on the surface of the plastic part that acts to poison the catalytic surface, interfering with metal deposition and often resulting in void formation. A simple water rinse is generally inadequate to remove the residue and the art has thus resorted to a further step of contact with a solution of a reducing agent although more chemistry is involved in removal of oxidant residue than simple reduction. Removal of permanganate residue with a reducing agent is disclosed in above referenced U.S. Pat. No. 4,610,895 to Tubergen and U.S. Pat. No. 6,645,557 to Joshi.
As is readily seen, many etching solutions have been suggested as a replacement for chromic acid in processes for preparing non-conductive substrates for metallization. However, none of these processes have proven satisfactory for various economic, performance and/or environmental reasons and thus none of these processes have achieved commercial success or been accepted by the industry as a suitable replacement for chromic acid etching.
The tendency for permanganate based solutions to form sludge and undergo self-decomposition has been noted. Under strongly acidic conditions, permanganate ions can react with hydrogen ions to produce manganese (II) ions and water according to the following reaction:4MnO4−+12-H+→4Mn2++6H2O+5O2  (1)
The manganese (II) ions formed by this reaction can then undergo further reaction with permanganate ions forming a sludge of manganese dioxide according to the following reaction:2MnO4−+2H2O+3Mn2+→5MnO2+4H+  (2)
Thus formulations based on strongly acidic permanganate solutions are intrinsically unstable irrespective of whether the permanganate ion is added by alkali metal salts of permanganate or is electrochemically generated in situ. In comparison to the currently used chromic acid etches, the poor chemical stability of acidic permanganate renders it effectively useless for large scale commercial application. Alkaline permanganate etches are more stable, and are widely used in the printed circuit board industry for etching epoxy based printed circuit boards, but alkaline permanganate is not an effective etchant for plastics such as ABS or ABS/PC. Thus, manganese (VII) is unlikely to gain widespread commercial acceptance as an etchant for these materials.
Other attempts to etch ABS without the use of chromic acid have included the use of electrochemically generated silver (II) or cobalt (III). For many years, it has been known that certain metals can be anodically oxidized to oxidation states which are highly oxidizing. For example, manganese (II) can be oxidized to permanganate (manganese VI), cobalt can be oxidized from cobalt (II) to cobalt (III) and silver can be oxidized from silver (I) to silver (II).
There is currently no suitable commercially successful etchant for plastics based on either permanganate (in either acid or alkaline form), on manganese in any other oxidation state or by using other acids or oxidants.
Thus, there remains a need in the art for an improved etchant for preparing plastic substrates for subsequent electroplating that does not contain chromic acid and that is commercially acceptable.